Two new proteins preferentially associated with membrane immunoglobulin D.

Abstract

The IgM and IgD classes of antigen receptor can perform different functions on B cells. However, so far no class‐specific components communicating with the cytoplasm have been found in the two antigen receptors. We have employed a new biotinylation protocol to search for intracellular membrane Ig‐associated proteins. Here we describe two proteins of 29 and 31 kDa that are associated with membrane IgD and to some extent with membrane IgM. The membrane IgM molecule is associated specifically with three proteins of 32, 37 and 41 kDa. The purification and sequencing of the two mIgD‐associated proteins revealed that they are novel proteins which are related to each other. These proteins may be the missing link between the antigen receptor and the cytoskeleton and may contribute to functional differences between membrane IgM and membrane IgD.

Bap31 Is an Itinerant Protein That Moves between the Peripheral Endoplasmic Reticulum (ER) and a Juxtanuclear Compartment Related to ER-associated Degradation

“…Bap31 is an integral ER membrane protein with three putative transmembrane domains (TMDs) and a dilysine motif at its C terminus (Kim et al, 1994), the latter of which is known to be able to act as a retrieval signal from post-ER compartments via COPI-coated vesicles (Cosson and Letourneur, 1994). Bap31 and its homologue Bap29 were originally discovered as B cell receptor-associated proteins (Kim et al, 1994). Although solid evidence demonstrated that Bap31 is involved in apoptosis (Breckenridge et al, 2003), accumulating evidence suggest that Bap31 in healthy cells functions as a cargo receptor for ER export of transmembrane proteins, such as cellubrevin, class I major histocompatibility complex (MHC) molecules, CFTR, membrane-bound immunoglobulin (Ig)G, tetraspanins, cytochrome P450 2C2, and the leukocyte integrin CD11b/CD18, some of which are well-known ERAD substrates (Annaert et al, 1997;Spiliotis et al, 2000;Lambert et al, 2001;Schamel et al, 2003;Paquet et al, 2004;Zen et al, 2004;Stojanovic et al, 2005;Ladasky et al, 2006; SzczesnaSkorupa and Kemper, 2006).…”

“…This article was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E07-08 -0781) on February 20, 2008. Bap31 is an integral ER membrane protein with three putative transmembrane domains (TMDs) and a dilysine motif at its C terminus (Kim et al, 1994), the latter of which is known to be able to act as a retrieval signal from post-ER compartments via COPI-coated vesicles (Cosson and Letourneur, 1994). Bap31 and its homologue Bap29 were originally discovered as B cell receptor-associated proteins (Kim et al, 1994).…”

“…Degradation of CFTR in yeast is independent of ER-to-Golgi traffic, but it is dependent on the function of Sar1p/COPII (Fu and Sztul, 2003), which are components of COPII-coated vesicles involved in ER export (Bonifacino and Glick, 2004;Lee et al, 2004). Bap31 is an integral ER membrane protein with three putative transmembrane domains (TMDs) and a dilysine motif at its C terminus (Kim et al, 1994), the latter of which is known to be able to act as a retrieval signal from post-ER compartments via COPI-coated vesicles (Cosson and Letourneur, 1994). Bap31 and its homologue Bap29 were originally discovered as B cell receptor-associated proteins (Kim et al, 1994).…”

A high-molecular-weight complex of membrane proteins BAP29/BAP31 is involved in the retention of membrane-bound IgD in the endoplasmic reticulum

“…These proteins were termed BCR-associated protein of 29 kDa (BAP29) and 31 kDa (BAP31) (20,21). The TM region of mIgD is sufficient for binding to BAP proteins, and this interaction involves polar amino acids within the TM region of mIgD (22).…”

“…The TM region of mIgD is sufficient for binding to BAP proteins, and this interaction involves polar amino acids within the TM region of mIgD (22). BAP31 and BAP29 are ubiquitously expressed polytopic membrane proteins, which are characterized by a hydrophobic Nterminal section with three putative TM regions and a Cterminal cytoplasmic tail containing predicted coiled-coil regions (20,22,23). BAP31 and BAP29 possess a C-terminal double-lysine motif (KKXX), which is known to localize proteins to the ER (24).…”